This invention is concerned with positron emission tomography systems.
Positron emission tomography (PET) is a type of nuclear imaging which is used in a variety of applications, particularly in medical research and diagnostic techniques. In a typical PET system, a radioactive compound, such as a fluorodeoxyglucose (a radiopharmaceutical), is administered to the patient being tested. The isotope is used to label a substance which circulates with the blood and which may be absorbed in certain tissues.
The isotopes used in PET systems decay be emitting a positively charged particle with the same mass as the electron (a positron) and the neutrino from the nucleus. In this process one of the protons in the nucleus becomes a neutron, so that its atomic number declines while its atomic weight remains constant. The positron is ejected with a kinetic energy of up to 2 MeV, depending on the isotope, and loses this energy by collisions as it travels within the body of the patient. When the positron reaches a thermal energy level, it interacts with an electron, resulting in mutual annihilation of the two particles. The rest mass of the two particles is transformed into two gamma rays of 511 KeV, which are characteristically emitted at 180.degree. with respect to each other.
The two gamma rays may be detected by suitable devices. These devices are normally scintillation detectors arranged in a precise geometrical pattern around the patient. A scintillation detector emits a light flash, with the intensity of the light proportional to the energy of the gamma ray, each time it absorbs gamma radiation, although this gamma radiation may or may not have arisen from the mutual annihilation of the positron and the electron.
Typically, a photomultiplier is used to convert the light flashes from the scintillation detector into an electrical charge pulse whose amplitude is proportional to the intensity of the light. If two detectors measure the energy of the gamma rays at about 511 KeV (i.e., equivalent to the mass of an electron at rest) and register this event almost simultaneously, it may be assumed that the origin of the radiation is on a straight line between the two detectors. Sufficient detectors are used in an arrangement designed to ensure that many coincident events may be imaged during the same time interval. The information from these detectors may then be processed by a computer using image reconstruction techniques in order to map the locations of the positron emitting isotope within the patient.
In prior art designs, PET imaging of a three dimensional object is typically obtained by using multiple rings of detectors, with each ring providing information from which a portion of the image corresponding to a "slice" through the patient can be developed. Three dimensional PET imaging would be preferable, but direct three dimensional PET imagers have so far been considered only theoretically because of the difficulty of obtaining sufficient readout of light from small, densely packed scintillation crystals by photomultiplier tubes.